Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.
Sterilizers are used in medical, food and packaging industries to prevent the transmission of agents such as spores, fungi and bacteria. A typical sterilizer creates a set of physical conditions in a sterilization chamber that can effectively kill nearly all of these transmissible agents.
One way of determining whether there has been sufficient exposure to the sterilant is to place test strips bearing a known micro-organism load in the sterilization chamber and to count the number of surviving micro organisms at the end of the sterilization process. That is time consuming, labour intensive, and impractical.
Alternatively, parametric monitoring can be employed in which measurements or controls are used to ensure that proper sterilization conditions are attained. Regulatory requirements for medical devices dictate that sterilizers have systems to verify the completion of a successful sterilization cycle. Time and temperature are two key parameters that need to be monitored for thermal sterilizers (autoclaves), and both of these are easily monitored with current technologies. In the case of sterilisers that use liquid chemical sterilants, regulatory requirements specify that the concentration or dosage of the sterilization chemistry delivered to the sterilization chamber must also be monitored. Once all the values for the necessary parameters are met, then it is possible to certify the articles as sterile and release them for use.
However, due to the corrosiveness of typical disinfection agents, measuring the dosage or concentration delivered is not a trivial matter, making certification of sterilisation difficult.
Sterilization processes which use an aerosol of microdroplets of a liquid sterilant in a gas stream (usually air) are known to be highly efficacious. These processes use, for example, an aerosol of droplets of hydrogen peroxide solution dispersed in an air stream which are kept in contact with an article to be sterilized for a predetermined time. These pose problems not only with the corrosive nature of the materials, but also the fact that a heterogeneous mixture (droplets in a gas) needs to be measured.
As used herein, the term “concentration” is used to refer to the amount or volume of active sterilising agent (such as hydrogen peroxide) relative to the amount or volume of inert carrier fluid (usually water) present. The term can be used in relation to a bulk liquid, to an individual aerosol particle, or to a collective group of aerosol particles generally, although it is not necessary that all particles in an aerosol have the same concentration, for example, if an aerosol arises from two different sources or if an aerosol has been partially modified in space or time.
The term “density” in relation to an aerosol refers to the amount of the total volume that is filled with aerosol particles. The density is a measure of a combination of aerosol droplet volume and the number of aerosol droplets per unit volume. Larger droplets or a higher number of droplets per unit area will both increase aerosol density, whereas smaller droplets or fewer droplets per unit volume will both decrease aerosol density.
The dosage of sterilant delivered is a function of the concentration, the density and the delivery time.
In order to verify sterilization, the dosage (i.e. the density delivered multiplied by the delivery time) of the liquid sterilant delivered to the sterilization chamber must be measured. If the article is exposed to too small a dose of sterilant, then sterilization cannot be certified and parametric release cannot take place. However, simply using a large excess of sterilant is not a practical option, since if the article is exposed to too high a dose, condensation of the aerosol droplets can take place on the surface of the article, leading to occlusion of the surface with used sterilant, which can result in reduced efficacy. Further, condensation can lead to the presence of residual sterilant on the apparatus to be sterilized. This can pose unacceptable risks to staff and patients, and the time needed to wash or dry the article may be longer than would otherwise be necessary, resulting in an unnecessarily long cycle time.
The present applicants have reasoned that if the concentration of sterilant in the solution being nebulised is known, then if the density of the aerosol droplets in the gas can be precisely determined (a quantified value of the mass of aerosol droplets in a given volume of the gas stream. e.g. grams of aerosol per m3 of gas) then the dose supplied to an article to be sterilized in a given time can be monitored. It would then be possible to use parametric monitoring to certify an article as sterile.
Hitherto there has been no simple, reliable and reproducible means for determining the density of an aerosol in a gas stream which was suitable to provide parametric monitoring data.
In the past aerosol density has been measured by optical means in which a gas flow containing an aerosol passes between a light source and a photo detector located on opposite sides of the gas flow path. A reduction in light detected by the photo detector is correlated with aerosol density by calibration and then used to indicate density. Initially unpublished attempts were made to measure changes in density optically and to combine those measurements with flow measurements. However the results were not acceptable for a variety of reasons.
Optical methods for estimating aerosol density suffer from a number of disadvantages. Generally, both light source brightness and photo detector sensitivity vary over time so that frequent recalibration of apparatus is required. Condensation on either the light source or detector lenses is a problem which requires the use of wipers or gas jets directed to prevent or remove condensates from the lens surface—a solution which introduces mechanical complexity and disturbs flow dynamics in the sterilization apparatus. Furthermore, reflection and diffraction of light by particles may cause light scattering rather than merely obscuration of part of the beam and result in measurements being influenced non linearly by variations in particle size or concentration.
In addition simple and economical optical methods are unable to measure the flow rate of the gas carrier. This would require some other flow rate measurement means and it would be advantageous if the aerosol density and gas stream flow rate could be measured with one transducer.
Alternative approaches avoiding direct aerosol measurement altogether involve the measurement of the sterilant liquid level in the nebuliser. By measuring the liquid level in the nebuliser before nebulisation, then measuring the liquid level after nebulisation, it is possible to calculate the total dosage of sterilant that has been nebulised. However, in practical terms, the amount of sterilant used is generally very small, meaning that a liquid level sensors need to be very accurate and repeatable to measure dosage. Devising a sensor to operate within the environment of a nebuliser that can accurately measure dosage levels is extremely difficult to achieve in practice.
There is a need for an improved method and apparatus for reliably determining the flow rate of a nebulant entrained in a gas stream over a range of flow rates and which is suitable for parametric monitoring. The invention is herein described primarily with reference to sterilization by means of a nebulant but the invention is not limited to use in sterilization, and those skilled in the art will appreciate that this method is suitable for any system where aerosol density and/or flow are desired to be known.
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.